Henrietta Swan Leavitt (July 4, 1868 – December 12, 1921) was an American astronomer who discovered the relation between the luminosity and the period of Cepheid variable stars. A graduate of Radcliffe College, Leavitt started working at the Harvard College Observatory as a "computer" in 1893, examining photographic plates in order to measure and catalog the brightness of stars. Though she received little recognition in her lifetime, it was her discovery that first allowed astronomers to measure the distance between the Earth and faraway galaxies. She explained her discovery: “A straight line can readily be drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods.”[1] After Leavitt's death, Edwin Hubble used the luminosity-period relation for Cepheids together with spectral shifts first measured by fellow astronomer Vesto Slipher at Lowell Observatory to determine that the Universe is expanding (see Hubble's law).

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Henrietta Swan Leavitt, the daughter of Congregational church minister George Roswell Leavitt[2] and his wife Henrietta Swan (Kendrick), was born in Lancaster, Massachusetts, a descendant of DeaconJohn Leavitt, an English Puritan tailor, who settled in the Massachusetts Bay Colony in the early seventeenth century.[3] (The family name was spelled Levett in early Massachusetts records.) She attended Oberlin College, and graduated from Radcliffe College, then called the Society for the Collegiate Instruction for Women, with a bachelor's degree in 1892. She studied a broad curriculum including classical Greek, fine arts, philosophy, analytic geometry and calculus.[4] It wasn't until her fourth year of college that Leavitt took a course in astronomy, in which she earned an A–.[5]:27 She then traveled in America and in Europe during which time she lost her hearing.[1]

In 1893, Leavitt began working at the Harvard College Observatory as one of the women human "computers" hired by Edward Charles Pickering to measure and catalog the brightness of stars as they appeared in the observatory's photographic plate collection. (In the early 1900s, women were not allowed to operate telescopes).[6] Because Leavitt had independent means, Pickering initially did not have to pay her. Later, she received $0.30 an hour for her work.[5]:32 She was reportedly “hard-working, serious-minded …, little given to frivolous pursuits and selflessly devoted to her family, her church, and her career.”[4]

Pickering assigned Leavitt to study "variable stars," whose luminosity varies over time. According to science writer Jeremy Bernstein, "variable stars had been of interest for years, but when she was studying those plates, I doubt Pickering thought she would make a significant discovery — one that would eventually change astronomy."[7] Leavitt noted thousands of variable stars in images of the Magellanic Clouds. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College,[8] noting that a few of the variables showed a pattern: brighter ones appeared to have longer periods. After further study, she confirmed in 1912 that the Cepheid variables with greater intrinsic luminosity did have longer periods, and that the relationship was quite close and predictable.[9]

Leavitt used the simplifying assumption that all of the Cepheids within each Magellanic Cloud were at approximately the same distances from Earth, so that their intrinsic brightness could be deduced from their apparent brightness (as measured from the photographic plates) and from the distance to each of the clouds. "Since the variables are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness."[10]

Her discovery is known as the "period-luminosity relationship": The logarithm of the period is linearly related to the star's average, intrinsic luminosity (which is defined as a logarithm of the amount of power radiated by the star in the visible spectrum). In Leavitt's words, taken from her study of 1,777 variable stars recorded on Harvard's photographic plates, "A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the [Cepheid] variables and their periods."[11][12]

Leavitt also developed and continued to refine the Harvard Standard for photographic measurements, a logarithmic scale that orders stars by brightness over 17 magnitudes. She initially analyzed 299 plates from 13 telescopes to construct her scale, which was accepted by the International Committee of Photographic Magnitudes in 1913.[13]

The period-luminosity relationship for Cepheids made them the first "standard candle" in astronomy, allowing scientists to compute the distances to galaxies too remote for stellar parallax observations to be useful. One year after Leavitt reported her results, Ejnar Hertzsprung determined the distance of several Cepheids in the Milky Way, and with this calibration the distance to any Cepheid could be accurately determined.[14]

Cepheids were soon detected in other galaxies, such as Andromeda (notably by Edwin Hubble in 1923–24), and they became an important part of the evidence that "spiral nebulae" are actually independent galaxies located far outside of our own Milky Way. Thus, Leavitt's discovery would forever change our picture of the Universe, as it prompted Harlow Shapley to move our Sun from the center of the galaxy in the "Great Debate" and Edwin Hubble to move our galaxy from the center of the Universe.

The accomplishments of the American astronomer Edwin Hubble, who established that the Universe is expanding, were also made possible by Leavitt's groundbreaking research. "If Henrietta Leavitt had provided the key to determine the size of the cosmos, then it was Edwin Powell Hubble who inserted it in the lock and provided the observations that allowed it to be turned," wrote David H. and Matthew D.H. Clark in their book Measuring the Cosmos.[15] To his credit, Hubble himself often said that Leavitt deserved the Nobel Prize for her work.[16]Gösta Mittag-Leffler of the Swedish Academy of Sciences tried to nominate her for that prize in 1924, only to learn that she had died of cancer three years earlier[17] (The Nobel Prize is not awarded posthumously).[18] Though she was paid only $10.50 per week, her discovery of a way to accurately measure distances on an inter-galactic scale paved the way for modern astronomy’s understanding of the structure and scale of the Universe.[4]

Henrietta Swan Leavitt working at her desk in the Harvard College Observatory[19]

Leavitt worked sporadically during her time at Harvard, often sidelined by health problems and family obligations. An illness contracted after her graduation from Radcliffe College rendered her increasingly deaf.[19] In 1921, when Harlow Shapley took over as director of the observatory, Leavitt was made head of stellar photometry. By the end of that year she had succumbed to cancer, and was buried in the Leavitt family plot at Cambridge Cemetery in Cambridge, Massachusetts.[5]:89

"Sitting at the top of a gentle hill," writes George Johnson in his biography of Leavitt, "the spot is marked by a tall hexagonal monument, on top of which (cradled on a draped marble pedestal) sits a globe. Her uncle Erasmus Darwin Leavitt and his family are also buried there, along with other Leavitts. A plaque memorializing Henrietta and her two siblings who died so young, Mira and Roswell, is mounted directly below the continent of Australia. Off to one side, and more often visited, are the graves of Henry and William James."[5]:90

Leavitt was a member of Phi Beta Kappa, the American Association of University Women, the American Astronomical and Astrophysical Society, the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers. Her early passing was seen as a tragedy by her colleagues for reasons that went beyond her scientific achievements. In an obituary her colleague, Solon I. Bailey, noted that "she had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning."[5]:28

Unaware of her death four years prior, the Swedish mathematician Gösta Mittag-Leffler considered nominating her for the 1926 Nobel Prize in Physics, and wrote to Shapley requesting more information on her work on Cepheid variables, offering to send her his monograph on Sofia Kovalevskaya. Shapley replied, let Mittag-Leffler know that Leavitt had died, and suggested that the true credit belonged to his (Shapley's) interpretation of her findings. She was never nominated, because the Nobel Prize is not awarded posthumously.[5]:118